#include<fstream>

using namespace std;

ifstream in("mountain.in");
ofstream out("mountain.out");

long long int n,ci,ans;
int d,x,y,temp;
int cha,shang,yu;

int a(int a,int b)
{
	cha=a-b;
	shang=cha/d;
	if(n-1-shang%2==0)
	{
		ci=(n-1-shang)/2+shang;
		ans=b+ci*d;
	}
	else
	{
		temp=cha-shang*d;
		ci=(n-2-shang)/2+shang;
		ans=b+ci*d+temp;
	}
	return ans;		
}


int main()
{

	in>>n>>d>>x>>y;
	if(x<y)
		out<<a(y,x);
	else
		out<<a(x,y);
	return 0;
}